In recent years, it is required to convert a solvent-type coating which contains an organic solvent into an aqueous coating with water as solvent in the field of coatings used in buildings and civil engineering structures. This is carried out for the purpose of reducing damage to the health of painting workers and habitants or reducing the atmospheric and environmental pollution, and use of the aqueous coating is increasing year by year.
Among the known coatings, there are coatings having performance at the same level with respect to weatherability, water resistance etc. as that of the solvent-type coatings. However, when attention is paid particularly to anti-staining properties, the level of the aqueous coatings even regarded as low staining is far from that of the solvent-type low-staining coatings at present.
A coating film of the aqueous coating tends to be poor in coating film hardness and highly stainable upon contacting with a staining substance, as compared with a coating film of a general solvent-type coatings. It follows that once a staining substance adheres thereto, its stain is often hardly removed from the surface of the coating film.
With respect to the solvent-type low-staining coating which can be used in buildings/civil engineering structures, a technology of compounding a coating with organosilicate and/or its condensate (referred to hereinafter as “organosilicate etc.”) is disclosed (WO94/06870). The technology of WO94/06870 comprises compounding an organic solvent-type coating with organosilicate etc., and treating its coating film with an acid thereby hydrophilizing the surface of the coating film to attain performance of making oily staining substance hardly adhering thereto and allowing adhered staining substance to be washed away with water droplets of rain or the like.
The organosilicate etc. used in the coating described in WO94/06870 supra have an alkoxysilyl group having reactivity with water, and upon addition to an aqueous coating, cannot regulate the hydrolysis condensation reaction of organosilicate etc., thus increasing the viscosity of the coating rapidly in a short time and further gelling the whole of the coating to cause a problem of hindering the coating operation, and also have a problem of generating aggregates after mixing the aqueous coating with organosilicate etc. because of poor compatibility therebetween. Due to this poor compatibility, a highly glossy aqueous coating called “glossy coating” particularly has a problem of extreme reduction in surface gloss. Even if organosilicate etc. are mixed with an aqueous coating and immediately used in forming a coating film, there is a disadvantage of failure to attain sufficient anti-staining properties. Particularly in an initial stage of formation of the coating film, there arises a problem of easier adhesion of staining substance due to the stickiness of organosilicate etc.
JP-A No. 2003-128998 describes an aqueous coating comprising a synthetic resin emulsion and aggregate such as ground products of natural stone, wherein organosilicate etc. are mixed as an anti-staining agent for the purpose of improving its anti-staining properties, but even this coating also has the same problem as in WO94/06870 supra. Particularly, the coating in JP-A No. 2003-128998 has another problem of deterioration in the vividness of the coating film.
As an aqueous coating for solving these problems, a coating wherein an alkoxysilane condensate having a polyoxyalkylene group is used as an anti-staining agent is known (WO99/05228). The aqueous anti-staining coating described in WO99/05228 is a coating of two-pack type consisting of an emulsion resin coating and an alkoxysilane condensate having a polyoxyalkylene group, wherein the compatibility of the alkoxysilane condensate with the emulsion resin coating has been improved.